1. Field of the Invention
The present invention relates generally to a precision overlay alignment system and, particularly, to proximity and projection lithography systems wherein ions, electrons or photons are utilized to transfer a high resolution pattern from a mask to a substrate.
2. Description of the Prior Art
There is a continuing desire to further reduce the minimum feature size of integrated circuits. This imposes a requirement of repeatably obtaining an exceedingly accurate degree of alignment between a mask and a substrate for each sequential lithographic step in the fabrication of an integrated circuit. For obtaining integrated circuits having feature sizes of below 1.0 .mu.m, an interferomic technique appears to possess the greatest likelihood of successful implementation.
D. C. Flanders et al., A New Interferometric Alignment Technique, Applied Physics Letters, Vol. 31, No. 7, Oct. 1, 1977, , pp. 426-428, describes the basic interferomic technique. Diffraction gratings of identical period are provided on the facing surfaces of a mask and substrate. The gratings are generally oriented with respect to one another such that they are parallel. A beam of laser light is directed normal to the diffraction grating planes with the result that diffracted light is returned at discrete angles from the incident laser beam, as may be determined by the equation: EQU n.lambda.=d(sin.phi..sub.n -sin.phi..sub.i),
wherein n is the diffraction group number, .lambda. is the incident beam wavelength, d is the grating period and .phi..sub.n and .phi..sub.i are the angles that the diffracted and incident beams make with respect to the normal of the diffraction grating planes. Only those beams suffering the smallest unit of an angular diffraction, those that have suffered a net first order diffraction, are utilized in the Flanders et al. technique. Further, the Flanders et al. technique utilizes the fact that the diffracted beams occur to either side of the incident laser beam. Thus, the first order diffraction group includes both plus (n=+1) and minus (n=-1) order beams. The plus first order diffraction group is composed of those beams that only suffer a plus first order diffraction from the mask grating (indicated as (1,0,0) in summary notation), or a sequential combination of diffractions such as a zeroth order diffraction from the mask grating, a plus first order diffraction from the substrate grating, and a zeroth order diffraction returning through the mask grating (0,1,0). The plus first order diffraction group may also include such beams that have sequentially suffered a zeroth order diffraction through the mask grating, a minus first order diffraction from the substrate grating, and a plus second order diffraction through the mask grating (0,-1,2). The result is an effective or net plus first order diffraction. Due to the symmetry of the gratings, a generally symmetrical set of beams are diffracted by the mask and wafer gratings so as to form both plus and minus first order diffraction groups.
The Flanders et al. technique measures the relative difference in the plus and minus first order diffraction group intensities to obtain an indication of the alignment of the mask and substrate gratings. For an in-plane displacement of the mask with respect to the substrate less than the period of the gratings, there is a corresponding variation in the relative intensities of the plus and minus first order diffraction groups due to the mutual interference between beams within each group. Ideally, there is a zero intensity difference between the plus and minus first order diffraction groups only when the mask and substrate diffraction grating lines are aligned. With the detection of sufficiently small intensity differences Flanders et al. concludes that alignment errors as small as 200 .ANG. can be detected.
There are, however, a number of inherent problems with the Flanders et al. technique. Of principal significance is that the Flanders et al. technique is highly sensitive to the specific spacing between the mask and substrate. In practical applications, this gap distance may vary due to bowing of the mask or wafer, or both, the tolerance errors in the machinery positioning the mask and substrate (particularly in such systems where there is a substrate step and repeat exposure sequence), and such transient perturbations as due to thermal, acoustic, and mechanical vibrations. These sources of gap distance variations further compound the simple fact that a gap exists at all. This interferomic technique relies on the interference between the respective first order diffracted group beams diffracted from the mask grating with those from the substrate grating. Since there is a spacing between the mask and substrate gratings, there is an inherent difference in the path length traversed by the respective mask and substrate grating diffracted beams. This introduces an effective phase retardation in those beams diffracted by the substrate grating. Thus, the interference that naturally occurs between the mask and substrate diffracted beams produces an equal intensity change but of opposite effective polarity in the respective plus and minus group beams. Consequently, when the mask and substrate gratings are in fact aligned, there will be an inherent difference in the intensities of the plus and minus first order diffraction group beams due to the presence of the gap.
While a constant difference in intensity might be appropriately dealt with, assuming the gap distance can be accurately and independently quantitized, the various causes of variations in the mask to substrate spacing are essentially random transients not subject to practical quantitization. Further, the variations in spacing are typically an appreciable fraction of the otherwise nominal gap distance. Consequently, there is no practical way to discriminate between mask and substrate grating alignment errors and the undesirable but nonetheless present variations in the mask to substrate spacing.
Another practical problem with the Flanders et al. technique is that it requires the diffraction efficiency of both the mask and substrate gratings to remain essentially constant and equal throughout the processing of the substrate. The efficiency of the mask grating may change due to the simple fact that different masks are utilized for the different sequential lithographic processing steps. However, the variations in mask grating efficiency alone are tolerable. In contrast, the efffective diffraction efficiency of the substrate grating is reduced by the simple fact that the substrate grating diffracted beams must pass twice through the mask grating. Further the substrate grating is utilized throughout the processing of the substrate and, therefore, is effectively exposed to the effects of all of the processing steps. Thus, the substrate grating efficiency degrades as the processing proceeds. Degradation of substrate grating efficiency results in a loss of its diffracted beam intensity and, thereby reduces the interference modulation of the plus and minus group beams. Consequently, for a given detector sensitivity, loss of grating efficiency directly increases the minimum limit of alignment error that can be detected.